We plan to continue to investigate the interaction of several antibiotics. Our finding that streptomycin blocks ribosomes shortly after initiation, but not at a later stage, has largely invalidated earlier evidence on the precise step that each antibiotics blocks, and we seek to define that step for each. The misreading effect of streptomycin on polysomal ribosomes will also be further analyzed. Our study of the reversibility of the reactions between runoff ribosomes, subunits, and IF-3 is continuing, particularly with a view to trying to reconcile our data with evidence of others that freshly released ribosomes exchange subunits more readily than 70S ribosomes that have accumulated. Work is in progress on attempts to synthesize derivatives of normal ligands that will form covalent bonds with adjacent groups on their binding site on the ribosome.